Notch1 competes with the amyloid precursor protein for gamma-secretase and down-regulates presenilin-1 gene expression

Presenilin 1 (PS1) is a critical component of the gamma-secretase complex, which is involved in the cleavage of several substrates including the amyloid precursor protein (APP) and Notch1. Based on the fact that APP and Notch are processed by the same gamma-secretase, we postulated that APP and Notch compete for the enzyme activity. In this report, we examined the interactions between APP, Notch, and PS1 using the direct gamma-secretase substrates, Notch 1 Delta extracellular domain (N1DeltaEC) and APP carboxyl-terminal fragment of 99 amino acids, and measured the effects on amyloid-beta protein production and Notch signaling, respectively. Additionally, we tested the hypothesis that downstream effects on PS1 expression may coexist with the competition phenomenon. We observed significant competition between Notch and APP for gamma-secretase activity; transfection with either of two direct substrates of gamma-secretase led to a reduction in the gamma-cleaved products, Notch intracellular domain or amyloid-beta protein. In addition, however, we found that activation of the Notch signaling pathway, by either N1 Delta EC or Notch intracellular domain, induced down-regulation of PS1 gene expression. This finding suggests that Notch activation directly engages gamma-secretase and subsequently leads to diminished PS1 expression, suggesting a complex set of feedback interactions following Notch activation.     

Cell surface presenilin-1 participates in the gamma-secretase-like proteolysis of Notch

Presenilin-1 (PS1), a polytopic membrane protein primarily localized to the endoplasmic reticulum, is required for efficient proteolysis of both Notch and beta-amyloid precursor protein (APP) within their trans- membrane domains. The activity that cleaves APP (called gamma-secretase) has properties of an aspartyl protease, and mutation of either of the two aspartate residues located in adjacent transmembrane domains of PS1 inhibits gamma-secretase processing of APP. We show here that these aspartates are required for Notch processing, since mutation of these residues prevents PS1 from inducing the gamma-secretase-like proteolysis of a Notch1 derivative. Thus PS1 might function in Notch cleavage as an aspartyl protease or di-aspartyl protease cofactor. However, the ER localization of PS1 is inconsistent with that hypothesis, since Notch cleavage occurs near the cell surface. Using pulse-chase and biotinylation assays, we provide evidence that PS1 binds Notch in the ER/Golgi and is then co-transported to the plasma membrane as a complex. PS1 aspartate mutants were indistinguishable from wild-type PS1 in their ability to bind Notch or traffic with it to the cell surface, and did not alter the secretion of Notch. Thus, PS1 appears to function specifically in Notch proteolysis near the plasma membrane as an aspartyl protease or cofactor.     

Positive and negative regulation of the gamma-secretase activity by nicastrin in a murine model

Nicastrin is a component of the gamma-secretase complex that has been shown to adhere to presenilin-1 (PS1), Notch, and APP. Here we demonstrate that Nicastrin-deficient mice showed a phenotype that is indistinguishable from PS1/PS2 double knock-out mice, whereas heterozygotes were healthy and viable. Fibroblasts derived from Nicastrin-deficient embryos were unable to generate amyloid beta-peptide and failed to release the intracellular domain of APP- or Notch1-Gal4-VP16 fusion proteins. Additionally, C- and N-terminal fragments of PS1 and the C-terminal fragments of PS2 were not detectable in Nicastrin-null fibroblasts, whereas full-length PS1 accumulated in null fibroblasts, indicating that Nicastrin is required for the endoproteolytic processing of presenilins. Interestingly, cells derived from Nicastrin heterozygotes produced relatively higher levels of amyloid beta-peptide whether the source was endogenous mouse or transfected human APP. These data demonstrate that Nicastrin is essential for the gamma-secretase cleavage of APP and Notch in mammalian cells and that Nicastrin has both positive and negative functions in the regulation of gamma-secretase activity.     

The role of presenilin-1 in the gamma-secretase cleavage of the amyloid precursor protein of Alzheimer's disease

Presenilin-1 (PS1) is required for the release of the intracellular domain of Notch from the plasma membrane as well as for the cleavage of the amyloid precursor protein (APP) at the gamma-secretase cleavage site. It remains to be demonstrated whether PS1 acts by facilitating the activity of the protease concerned or is the protease itself. PS1 could have a gamma-secretase activity by itself or could traffic APP and Notch to the appropriate cellular compartment for processing. Human APP 695 and PS1 were coexpressed in Sf9 insect cells, in which endogenous gamma-secretase activity is not detected. In baculovirus-infected Sf9 cells, PS1 undergoes endoproteolysis and interacts with APP. However, PS1 does not cleave APP in Sf9 cells. In CHO cells, endocytosis of APP is required for Abeta secretion. Deletion of the cytoplasmic sequence of APP (APPDeltaC) inhibits both APP endocytosis and Abeta production. When APPDeltaC and PS1 are coexpressed in CHO cells, Abeta is secreted without endocytosis of APP. Taken together, these results conclusively show that, although PS1 does not cleave APP in Sf9 cells, PS1 allows the secretion of Abeta without endocytosis of APP by CHO cells.     

The Large Hydrophilic Loop of Presenilin 1 Is Important for Regulating ��-Secretase Complex Assembly and Dictating the Amyloid �� Peptide (A��) Profile without Affecting Notch Processing

γ-Secretase is an enzyme complex that mediates both Notch signaling and β-amyloid precursor protein (APP) processing, resulting in the generation of Notch intracellular domain, APP intracellular domain, and the amyloid β peptide (Aβ), the latter playing a central role in Alzheimer disease (AD). By a hitherto undefined mechanism, the activity of γ-secretase gives rise to Aβ peptides of different lengths, where Aβ42 is considered to play a particular role in AD. In this study we have examined the role of the large hydrophilic loop (amino acids 320–374, encoded by exon 10) of presenilin 1 (PS1), the catalytic subunit of γ-secretase, for γ-secretase complex formation and activity on Notch and APP processing. Deletion of exon 10 resulted in impaired PS1 endoproteolysis, γ-secretase complex formation, and had a differential effect on Aβ-peptide production. Although the production of Aβ38, Aβ39, and Aβ40 was severely impaired, the effect on Aβ42 was affected to a lesser extent, implying that the production of the AD-related Aβ42 peptide is separate from the production of the Aβ38, Aβ39, and Aβ40 peptides. Interestingly, formation of the intracellular domains of both APP and Notch was intact, implying a differential cleavage activity between the ϵ/S3 and γ sites. The most C-terminal amino acids of the hydrophilic loop were important for regulating APP processing. In summary, the large hydrophilic loop of PS1 appears to differentially regulate the relative production of different Aβ peptides without affecting Notch processing, two parameters of significance when considering γ-secretase as a target for pharmaceutical intervention in AD.     

Low density lipoprotein receptor-related protein (LRP) interacts with presenilin 1 and is a competitive substrate of the amyloid precursor protein (APP) for gamma-secretase

Presenilin 1 (PS1) is a critical component of the gamma-secretase complex, which is involved in the cleavage of several substrates including the amyloid precursor protein (APP) and the Notch receptor. Recently, the low density receptor-related protein (LRP) has been shown to be cleaved by a gamma-secretase-like activity. We postulated that LRP may interact with PS1 and tested its role as a competitive substrate for gamma-secretase. In this report we show that LRP colocalizes and interacts with endogenous PS1 using coimmunoprecipitation and fluorescence lifetime imaging microscopy. In addition, we found that gamma-secretase active site inhibitors do not disrupt the interaction between LRP and PS1, suggesting that the substrate associates with a gamma-secretase docking site located in close proximity to PS1. This is analogous to APP-gamma-secretase interactions. Finally, we show that LRP competes with APP for gamma-secretase activity. Overexpression of a truncated LRP construct consisting of the C terminus, the transmembrane domain, and a short extracellular portion leads to a reduction in the levels of the Abeta40, Abeta42, and p3 peptides without changing the total level of APP expression. In addition, transfection with the beta-chain of LRP causes an increase in uncleaved APP C-terminal fragments and a concomitant decrease in the signaling effects of the APP intracellular domain. In conclusion, LRP is a PS1 interactor and can compete with APP for gamma-secretase enzymatic activity.     

Presenilin 1 mutations activate gamma 42-secretase but reciprocally inhibit epsilon-secretase cleavage of amyloid precursor protein (APP) and S3-cleavage of notch

The presenilin 1 (PS1) and presenilin 2 (PS2) proteins are necessary for proteolytic cleavage of the amyloid precursor protein (APP) within its transmembrane domain. One of these cleavage events (termed gamma-secretase) generates the C-terminal end of the Abeta-peptide by proteolysis near residue 710 or 712 of APP(770). Another event (termed gamma-like or epsilon-secretase cleavage) cleaves near residue 721 at approximately 2-5 residues inside the cytoplasmic membrane boundary to generate a series of stable, C-terminal APP fragments. This latter cleavage is analogous to S3-cleavage of Notch. We report here that specific mutations in the N terminus, loop, or C terminus of PS1 all increase the production of Abeta(42) but cause inhibition of both epsilon-secretase cleavage of APP and S3-cleavage of Notch. These data support the hypothesis that epsilon-cleavage of APP and S3-cleavage of Notch are similar events. They also argue that, although both the gamma-site and the epsilon-site cleavage of APP are presenilin-dependent, they are likely to be independent catalytic events.     

Rac1 changes the substrate specificity of gamma-secretase between amyloid precursor protein and Notch1

Beta amyloid peptide is generated from amyloid precursor protein (APP) by proteolytic cleavage of β- and γ-secretases, and plays a critical role in the pathogenesis of Alzheimer’s disease. Since γ-secretase cleaves several proteins including APP and Notch in a number of cell types, it is important to understand the conditions determining γ-secretase substrate specificity. In the present study, inhibition of Rac1 attenuated γ-secretase activity for APP, resulting in decreased production of the APP intracellular domain but accumulated C-terminal fragments (APP-CTF). In contrast, Rac1 inhibitor, NSC23766 increased production of the Notch1 intracellular domain but slightly decreased the ectodomain-shed form of Notch1 (NotchΔE). To elucidate the mechanism underlying these observations, we performed co-immunoprecipitation experiments to analyze the interaction between Rac1 and presenilin1 (PS1), a component of the γ-secretase complex. Inhibition of Rac1 enhanced its interaction with PS1. Under the same condition, PS1 interacted more strongly with NotchΔE than with APP-CTF. Our results suggested that PS1 determines the preferred substrate for γ-secretase between APP and Notch1, depending on the activation status of Rac1.     

Presenilin clinical mutations can affect gamma-secretase activity by different mechanisms

Mutations in human presenilin (PS) genes cause aggressive forms of familial Alzheimer's disease. Presenilins are polytopic proteins that harbour the catalytic site of the gamma-secretase complex and cleave many type I transmembrane proteins including beta-amyloid precursor protein (APP), Notch and syndecan 3. Contradictory results have been published concerning whether PS mutations cause 'abnormal' gain or (partial) loss of function of gamma-secretase. To avoid the possibility that wild-type PS confounds the interpretation of the results, we used presenilin-deficient cells to analyse the effects of different clinical mutations on APP, Notch, syndecan 3 and N-cadherin substrate processing, and on gamma-secretase complex formation. A loss in APP and Notch substrate processing at epsilon and S3 cleavage sites was observed with all presenilin mutants, whereas APP processing at the gamma site was affected in variable ways. PS1-Delta9 and PS1-L166P mutations caused a reduction in beta-amyloid peptide Abeta40 production whereas PS1-G384A mutant significantly increased Abeta42. Interestingly PS2, a close homologue of PS1, appeared to be a less efficient producer of Abeta than PS1. Finally, subtle differences in gamma-secretase complex assembly were observed. Overall, our results indicate that the different mutations in PS affect gamma-secretase structure or function in multiple ways.     

Functional domains in presenilin 1: the Tyr-288 residue controls gamma-secretase activity and endoproteolysis

Processing of the Alzheimer amyloid precursor protein (APP) into the amyloid beta-protein and the APP intracellular domain is a proteolysis event mediated by the gamma-secretase complex where presenilin (PS) proteins are key constituents. PS is subjected to an endoproteolytic cleavage, generating a stable heterodimer composed of an N-terminal and a C-terminal fragment. Here we aimed at further understanding the role of PS in endoproteolysis, in proteolytic processing of APP and Notch, and in assembly of the gamma-secretase complex. By using a truncation protocol and alanine scanning, we identified Tyr-288 in the PS1 N-terminal fragment as critical for PS-dependent intramembrane proteolysis. Further mutagenesis of the 288 site identified mutants differentially affecting endoproteolysis and gamma-secretase activity. The Y288F mutant was endoproteolyzed to the same extent as wild type PS but increased the amyloid beta-protein 42/40 ratio by approximately 75%. In contrast, the Y288N mutant was also endoproteolytically processed but was inactive in reconstituting gamma-secretase in PS null cells. The Y288D mutant was deficient in both endoproteolysis and gamma-secretase activity. All three mutant PS1 molecules were incorporated into gamma-secretase complexes and stabilized Pen-2 in PS null cells. Thus, mutations at Tyr-288 do not affect gamma-secretase complex assembly but can differentially control endoproteolysis and gamma-secretase activity.     

A sensitive and quantitative assay for measuring cleavage of presenilin substrates.

The presenilin (PS) proteins are components of the gamma-secretase activity, which is central in the pathogenesis of Alzheimer's disease. Here we present a novel cell-based reporter gene assay for the quantification of PS-controlled gamma-secretase cleavage of the Alzheimer amyloid precursor protein (APP). We show that this assay offers several advantages, including increased sensitivity and specificity, improved quantification of cleavage, and simultaneous detection of all gamma-secretase cleavages in APP. Furthermore, the APP assay can be used in parallel with a similar assay that records gamma-secretase cleavage of a Notch receptor. The use of these assays to analyze the effects of two known gamma-secretase inhibitors and postulated PS active site mutants on APP and Notch processing demonstrated that inhibitors and mutants that differently affect Notch and APP cleavage can be identified rapidly. The possibility in using these assays for high throughput screening of candidate gamma-secretase inhibitors for APP and Notch in parallel opens up new vistas to systematically search for novel inhibitors that selectively block APP cleavage while not affecting Notch signaling.     

Minor contribution of presenilin 2 for γ-secretase activity in mouse embryonic fibroblasts and adult mouse brain

γ-Secretase plays an important function in the development of Alzheimer disease, since it participates in the production of the toxic amyloid β-peptide (Aβ) from the amyloid precursor protein (APP). Besides APP, γ-secretase cleaves many other substrates resulting in adverse side effects when γ-secretase inhibitors are used in clinical trials. γ-Secretase is a membrane bound protein complex consisting of at least four subunits, presenilin (PS), nicastrin, Aph-1 and Pen-2. PS and Aph-1 exist as different homologs (PS1/PS2 and Aph-1a/Aph-1b, respectively), which generates a variation in complex composition. PS1 and PS2 appears to have distinct roles since PS1 is essential during embryonic development whereas PS2 deficient mice are viable with a mild phenotype. The molecular mechanism behind this diversity is, however, largely unknown. In order to investigate whether PS1 and PS2 show different substrate specificity, we used PS1 or PS2 deficient mouse embryonic fibroblasts to study the processing on the γ-secretase substrates APP, Notch, N-cadherin, and ephrinB. We found that whereas depletion of PS1 severely affected the cleavage of all substrates, the effect of PS2 depletion was minor. In addition, less PS2 was found in active γ-secretase complexes. We also studied the effect of PS2 depletion in adult mouse brain and, in concordance with the results from the mouse embryonic fibroblasts, PS2 deficiency did not alter the cleavage of the two most important substrates, APP and Notch. In summary, this study shows that the contribution of PS2 on γ-secretase activity is of less importance, explaining the mild phenotype of PS2-deficient mice.     

Specificity of presenilin‐1‐ and presenilin‐2‐dependent γ‐secretases towards substrate processing

The two presenilin‐1 (PS1) and presenilin‐2 (PS2) homologs are the catalytic core of the γ‐secretase complex, which has a major role in cell fate decision and Alzheimer's disease (AD) progression. Understanding the precise contribution of PS1‐ and PS2‐dependent γ‐secretases to the production of β‐amyloid peptide (Aβ) from amyloid precursor protein (APP) remains an important challenge to design molecules efficiently modulating Aβ release without affecting the processing of other γ‐secretase substrates. To that end, we studied PS1‐ and PS2‐dependent substrate processing in murine cells lacking presenilins (PSs) (PS1KO, PS2KO or PS1‐PS2 double‐KO noted PSdKO) or stably re‐expressing human PS1 or PS2 in an endogenous PS‐null (PSdKO) background. We characterized the processing of APP and Notch on both endogenous and exogenous substrates, and we investigated the effect of pharmacological inhibitors targeting the PSs activity (DAPT and L‐685,458). We found that murine PS1 γ‐secretase plays a predominant role in APP and Notch processing when compared to murine PS2 γ‐secretase. The inhibitors blocked more efficiently murine PS2‐ than murine PS1‐dependent processing. Human PSs, especially human PS1, expression in a PS‐null background efficiently restored APP and Notch processing. Strikingly, and contrary to the results obtained on murine PSs, pharmacological inhibitors appear to preferentially target human PS1‐ than human PS2‐dependent γ‐secretase activity.     

Regulation of amyloid precursor protein processing by presenilin 1 (PS1) and PS2 in PS1 knockout cells

The presenilin 1 (PS1) and PS2 proteins are thought to play roles in processing of amyloid precursor protein (APP), but the nature of this role is not fully understood. Recent studies have shown that PS1 is necessary for cleavage of APP at the gamma-secretase site. We now show that PS1 and PS2 participate in other aspects of APP processing. Fibroblasts generated from PS1 knockout mice have increased levels of the APP cleavage products, secreted APP (APPs), and APP C-terminal fragments, but lower secretion of APPs and Abeta. We have also observed that loss of PS1 prevents protein kinase C or extracellular regulated kinase from increasing production of the APP cleavage products, APPs, and APP C-terminal fragments. Transfection of PS1 -/- cells with PS1 restores the responsiveness of APP processing to protein kinase C and extracellular regulated kinase. This suggests that the changes in APP processing in PS1 -/- cells result strictly from the absence of PS1. Transfection of PS1 -/- cells with PS2 is also able to correct the deficits in APP secretion, which suggests that the PS2 also has the ability to regulate APP processing. Finally, transfection of the truncated PS2 construct, Alg3, into cells lacking PS1 increases APP C-terminal fragments. This suggests that Alg3 can interfere with the processing of APP by PS2. These data point to roles for both PS1 and PS2 in regulating APP processing and suggest that the role of these proteins also includes coupling APP to signal transduction pathways.     

Identification of presenilin 1-selective γ-secretase inhibitors with reconstituted γ-secretase complexes

Accumulation of the β-amyloid (Aβ) peptides is one of the major pathologic hallmarks in the brains of Alzheimer's disease (AD) patients. Aβ is generated by sequential proteolytic cleavage of the amyloid precursor protein (APP) catalyzed by β- and γ-secretases. Inhibition of Aβ production by γ-secretase inhibitors (GSIs) is thus being pursued as a target for treatment of AD. In addition to processing APP, γ-secretase also catalyzes proteolytic cleavage of other transmembrane substrates, with the best characterized one being the cell surface receptor Notch. GSIs reduce Aβ production in animals and humans but also cause significant side effects because of the inhibition of Notch processing. The development of GSIs that reduce Aβ production and have less Notch-mediated side effect liability is therefore an important goal. γ-Secretase is a large membrane protein complex with four components, two of which have multiple isoforms: presenilin (PS1 or PS2), aph-1 (aph-1a or aph-1b), nicastrin, and pen-2. Here we describe the reconstitution of four γ-secretase complexes in Sf9 cells containing PS1--aph-1a, PS1--aph-1b, PS2--aph-1a, and PS2--aph-1b complexes. While PS1--aph-1a, PS1--aph-1b, and PS2--aph-1a complexes displayed robust γ-secretase activity, the reconstituted PS2--aph-1b complex was devoid of detectable γ-secretase activity. γ-Secretase complexes containing PS1 produced a higher proportion of the toxic species Aβ42 than γ-secretase complexes containing PS2. Using the reconstitution system, we identified MRK-560 and SCH 1500022 as highly selective inhibitors of PS1 γ-secretase activity. These findings may provide important insights into developing a new generation of γ-secretase inhibitors with improved side effect profiles.     

Characterization of the reconstituted gamma-secretase complex from Sf9 cells co-expressing presenilin 1, nicastrin [correction of nacastrin], aph-1a, and pen-2

Gamma-secretase catalyzes the proteolytic processing of a number of integral membrane proteins, including amyloid precursor protein (APP) and Notch. The native gamma-secretase is a heterogeneous population of large membrane protein complexes containing presenilin 1 (PS1) or presenilin 2 (PS2), aph-1a or aph-1b, nicastrin, and pen-2. Here we report the reconstitution of a gamma-secretase complex in Sf9 cells by co-infection with baculoviruses carrying the PS1, nicastrin, pen-2, and aph-1a genes. The reconstituted enzyme processes C99 and the Notch-like substrate N160 and displays the characteristic features of gamma-secretase in terms of sensitivity to a gamma-secretase inhibitor, upregulation of Abeta42 production by a familial Alzheimer's disease (FAD) mutation in the APP gene, and downregulation of Notch processing by PS1 FAD mutations. However, the ratio of Abeta42:Abeta40 production by the reconstituted gamma-secretase is significantly higher than that of the native enzyme from 293 cells. Unlike in mammalian cells where PS1 FAD mutations cause an increase in Abeta42 production, PS1 FAD missense mutations in the reconstitution system alter the cleavage sites in the C99 substrate without changing the Abeta42:Abeta40 ratio. In addition, PS1DeltaE9 is a loss-of-function mutation in both C99 and N160 processing. Reconstitution of gamma-secretase provides a homogeneous system for studying the individual gamma-secretase complexes and their roles in Abeta production, Notch processing and AD pathogenesis. These studies may provide important insight into the development of a new generation of selective gamma-secretase inhibitors with an improved side effect profile.     

A domain at the C-terminus of PS1 is required for presenilinase and gamma-secretase activities

The structural requirements for presenilin (PS) to produce active presenilinase and gamma-secretase enzymes are poorly understood. Here we investigate the role the cytoplasmic C-terminal region of PS1 plays in PS1 activity. Deletion or addition of residues at the PS C-terminus has been reported to inhibit presenilinase endoproteolysis of PS and alter gamma-secretase activity. In this study, we use a sensitive assay in PS1/2KO MEFs to define a domain at the extreme C-terminus of PS1 that is essential for both presenilinase and gamma-secretase activities. Progressive deletion of the C-terminus demonstrated that removal of nine residues produces a PS1 molecule (458ST) that lacks both presenilinase processing and gamma-secretase cleavage of Notch and APP substrates. In contrast, removal of four or five residues had no effect (462ST, 463ST), while intermediate truncations partially inhibited PS1 activity. The 458ST mutant was unable to replace endogenous wtPS1 in HEK293 cells. Although 458ST was able to form a gamma-secretase complex, this complex was not matured, illustrated by mutant PS1 instability, lack of endoproteolysis, and little production of mature Nicastrin. These data indicate that the C-terminal end of PS1 is essential for Nicastrin trafficking and modification as well as the replacement of endogenous PS1 by PS1 transgenes.